In an image forming apparatus, such as a copying machine, a facsimile machine or a printer, a laser beam emitted from a semiconductor laser (e.g., a laser diode) is scanned over the surface of a photosensitive drum (which serves as an image bearing member), so as to form an electrostatic latent image on the surface of the photosensitive drum. The electrostatic latent image is developed with a developing agent (toner), and then transferred onto a sheet.
The laser beam emitted from the laser diode falls on a galvano-mirror, by which it is reflected and incident on a polygonal mirror. The laser beam incident on the polygonal mirror is reflected, and it is scanned over the surface of the photosensitive drum in the axial direction thereof in accordance with the rotation of the polygonal mirror. A scan performed in the axial direction of the photosensitive drum is generally referred to as a main scan. This main scan is repeatedly executed in accordance with the rotation of the photosensitive drum. The direction in which the main scan repeatedly executed on the photosensitive drum is shifted (i.e., the direction orthogonal to the direction of the main scan) is generally referred to a sub-scan direction.
The position of the laser beam can be adjusted in the sub-scan direction by use of the galvano-mirror.
The polygonal mirror is driven by a motor. The speed of the main scan is determined by the speed of this motor. In order to optimally determine the magnification of the image formation performed in the direction of the main scan, it is important to control the speed of the main scan to be an appropriate value.
In the case of a complex-type image forming apparatus which has both a copy mode and a print mode as its image-forming modes, the speed of the motor of the polygonal mirror (i.e., the speed of the main scan of the laser beam) is control to be different between the copy mode and the print mode.
In the copy mode, an image on a document is optically read, and the laser beam is modulated in accordance with the read image data. In the copy mode, therefore, the speed of the motor of the polygonal mirror is determined in consideration of the movement of the optical system used for optically reading a document image.
In the print mode, the laser beam is modulated in accordance with image data that are externally input. In the print mode, therefore, the speed of the motor of the polygonal mirror is determined without reference to the movement of the optical system.
Speed-determining data, which determine the motor speed in the copy mode and that in the print mode, are stored in a memory. On the basis of the speed-determining data in the memory, the speed of the motor of the polygonal mirror is controlled.
In the complex-type image forming apparatus described above, the laser beam has to be adjusted in consideration of the phenomenon that the speed of the motor of the polygonal mirror changes when the image-forming mode is switched from one to the other. It should be noted, however, that the adjustment of the laser beam inevitably results in a stand-by state before the execution of image formation.